Agricultural and Environmental
Impacts of Drainage

Introduction

Water is a vital resource for
agriculture, Indiana’s largest
industry. The plentiful supply of water in most years helps make Indiana
one of the nation’s leading agricultural states.However, at certain times of the year excess
water can prevent timely farm operations, restrict plant root growth, and increase
erosion.

The purpose of agricultural
drainage is to remove excess water from the soil in order to enhance crop
production. In some soils, the natural drainage processes are sufficient for
growth and production of agricultural crops, but in many other soils,
artificial drainage is needed for efficient agricultural production. About 50%
of Indiana’s cropland is
artificially drained. This percentage is among the
highest in the U.S.
The percentage of cropland that has subsurface or tile drainage in Indiana
(35%) is the highest in the nation. Most of the more than 8 million acres in Indiana
with drainage improvements would not be productive without the ditches or tiles
that remove excess water from the field. Drainage is therefore a critical
component of agricultural production in Indiana.

While enabling Indiana
farmers to produce outstanding yields, drainage has led to the loss of wetlands
that support wildlife and improve water quality. Tile drains also present a
direct flowpath for nitrate loss, a potential water
quality concern. Public concern over the loss of wetlands and water quality
effects of drainage has led to serious questions about the impact of drainage
improvements on Indiana’s water
resources and environment. This publication discusses the need for drainage in Indiana,
explains how agricultural drainage works, and addresses the environmental
effects of drainage.

Why is drainage needed?

Average annual precipitation in Indiana
ranges from 36 inches in the northeast to 44 inches in the southwest. Only
about two-thirds of this is used by crops. Monthly precipitation remains fairly
constant throughout the year, while evapotranspiration
(a combination of evaporation from soil and transpiration from the crop), changes
with the season. From January to May, and from October to December,
precipitation is considerably greater than evapotranspiration,
creating a water surplus. Crop water needs exceed precipitation only in July,
August, and September.The surplus of precipitation
over evapotranspiration results in excess water in
the crop root zone.

If the water table (the uppermost
depth at which water moves freely in the soil) is too high, crop growth is
reduced. The water table can be thought of as the depth to which water will
rise in a well or a hole dug in the ground. Some low-lying soils have permanent
high water tables. Other soils may be poorly drained because of seepage from
upslope areas, or because they are in a depressional
area with no outlet. Some soils have a slowly permeable subsurface layer, which
restricts vertical drainage and leads to a high water table during some
portions of the year. More detailed information on soils and drainage can be
found in AY-301, “Wet Soils of Indiana.”

How does drainage benefit crops?

Soils consist of solid particles
(sand, silt, clay, and decomposing plant materials) and the pore spaces between
the solid particles, which may be filled with water or air or both. Plant roots
need oxygen to grow. When the soil is saturated with water, the plant roots
will survive for a short time by using the oxygen dissolved in the water. With
prolonged wetness, however, the oxygen is depleted and roots die due to a lack
of oxygen. Tile drainage lowers the water table, making room for air to move
back into the soil and replenish oxygen to the roots. Thus, a major function of
drainage is to improve aeration for root growth.

In a poorly drained soil, root
growth is restricted by the high water table early in the season, such that
when the water table drops rapidly during mid-season, further root growth and
hence crop growth is impaired. In the drained soil, however, the root system
develops more fully in the spring, enabling the plant to have access to deeper
water in the dry periods of mid-summer.

Another symptom of poor drainage
that can be readily seen in some years is an overall yellow appearance in the
green vegetation, especially for corn and wheat. Although the yellow color may
have several different causes, one of the most common is a nitrogen deficiency
in the plant. This may be due to an oxygen shortage causing poor uptake of
nutrients by the roots, or it may be due to denitrification
(conversion of nitrate to nitrogen gas), a loss of needed nitrogen from the
soil. These yellow areas in a field can be useful indicators of places where
additional drainage improvement might be needed.

Improved drainage is also vital
for timely field operations in the spring. It is estimated that corn yields are
reduced 1-2 bushels per acre for each day after May 10 that corn is planted in Indiana.
Since driving on a wet field is detrimental to soil structure, the soil must be
adequately dry before planting can take place. Improved drainage allows the
farmer to get into the field several days to several weeks earlier than would
be possible without drainage.

How does drainage work?

Two types of drainage improvements
are commonly used in Indiana:
surface and subsurface. Often a combination of these two is used, which usually
maximizes drainage benefits.

Surface drainageis the removal of water that collects
on the land surface. Many fields have low spots or depressions where water
ponds. Surface drainage techniques such as land leveling, constructing surface
inlets to subsurface drains, and the construction of shallow ditches or
waterways can allow the water to leave the field rather than causing prolonged
wet areas.

Subsurface drainage
removes excess water from the soil profile, usually through a network of
perforated tubes installed 2 to 4 feet below the soil surface. These tubes are
commonly called “tiles” because formerly they were made from short lengths of
clay pipes known as tiles. Water would seep into the small spaces between the
tiles. Today the most common type of “tile” is actually corrugated plastic
tubing with small perforations to allow water entry. When the water table in
the soil is higher than the tile, water flows into the tubing, either through
holes in the plastic tube or through the small cracks between adjacent clay
tiles. This lowers the water table to the depth of the tile over the course of
several days.

Drain tiles allow excess water to
leave the field, but once the water table has been lowered to the elevation of
the tiles, no more water flows through the tiles. In most years, drain tiles
are not flowing between June and October.

Where does the water go?

The water is carried through the
drain to an appropriate outlet, usually a stream or a ditch. The outlet is one
of the most important considerations in planning and installing a drainage
system. Indiana has many flat
areas where it is often difficult to find an outlet sufficiently low to drain
the field. In a few cases, water is pumped up to a ditch or stream, although
this is much more expensive than using gravity alone.

Construction of drains that are
shared by many landowners was an important process in agricultural development
in Indiana. Certain drains have
been designated as regulated drains by the County Drainage Board (or the
Commissioners Court or Circuit Court of each county prior to 1965.) Maintenance
of these regulated drains today is the responsibility of the Drainage Board,
which consists of three members appointed by the CountyCommissioners, advised by the CountySurveyor. The Drainage Board is
responsible for construction and maintenance of drains, and to make judgments
when there is a conflict between landowners that may use the same drain.

Ultimately, the water drained from
agricultural field in Indiana
flows to rivers and streams that carry it either to the Great Lakes
(the northern 10% of the state) or the Mississippi River
(the rest of the State).

How are drainage systems designed?

Designing and installing a drainage
system is a complex process. Every field is unique and usually requires an
individual design. Drainage depends on topography, crops that will be grown on
the field, and soil type. Every soil type has different properties that affect
its drainage. Scientists and engineers have developed recommendations for
drainage depth and spacing in each soil type in Indiana
based on years of experience and knowledge of soil properties. These
recommendations are given in AY-300, Drainage Recommendations for Indiana Soils.

Drainage contractors use these
recommendations, along with principles of sound drainage design, to design
drainage systems that economically and effectively drain a particular field.

Don’t we need to protect wetlands?

There is no doubt that much of the
Indiana landscape consisted of
wetlands before large-scale drainage began in the 19th century. Stories about
wagons sinking in the swampy ground, constant swarms of mosquitoes, and even
malaria can be found in many accounts of early settlement of Indiana.
About 85% of Indiana’s original
wetlands have been drained. Although the public health and economic benefits
resulting from the draining of these wetlands over the last 150 years are
clear, there have also been negative impacts on the environment.Wetlands have an important hydrologic
function in regulating water flow and improving water quality, as well as
providing habitat for water-based wildlife. Recognition of the
their valuable functions has changed the way society thinks about and
protects wetlands.

The 1985 Farm Bill, or Food
Security Act, made a dramatic change in the way the U.S.
treats wetlands. The provision commonly known as “swampbuster”
made anyone who converted land from wetlands to agriculture ineligible for
federal farm benefits. Land converted prior to 1985 can remain in agriculture,
but further conversion of wetlands is heavily restricted. More information on
drainage and wetlands can be found inID-xx,
“Wetland Regulations in Indiana.”

Drainage improvements today are
therefore very rarely for the purpose of converting existing wetlands to
agricultural production, but are usually aimed at making existing agricultural
land more productive.Some fields have
drain tiles that were installed 100 or more years ago, and are broken or
plugged. In many fields, only a few of the wettest spots were originally
drained, while the entire field would benefit from improved drainage. More
tiles are often added to improve drainage efficiency, with the goal of
increasing production.

How do tile drains affect water quality?

Drainage has both positive and
negative effects on water quality. In general, land that has good subsurface
drainage has less surface runoff, erosion, and phosphorus transport than
equivalent land without drainage improvements or with only surface drainage.
Figure 1 shows a hydrograph (a graph of water flow as a function of time) from
two fields that are similar in every way, except that one has good subsurface
drainage while the other has poor surface drainage. The total flow from each
one is about the same, but the field with poor subsurface drainage has a peak
flow rate more than twice as high as the other. Higher peak flows usually
result in more erosion, so sediment problems are usually reduced by good
subsurface drainage. Phosphorus, which moves with eroded soil, is also reduced
when more water flows with subsurface drainage rather than as surface runoff.

Good subsurface

drainage

Poor subsurface

drainage

Figure 1: Flow from a watershed
with poor drainage and a similar watershed with good subsurface drainage.

Nitrate movement does not depend
on surface runoff, however. Because it is very soluble, it flows readily with
water through the soil and into tile lines. Nitrate concentration often increases
with improved subsurface drainage. For example, the nitrate concentration
measured in the watersheds shown in Figure 3 was nearly three times higher in
the watershed with good subsurface drainage. Nitrate flow from subsurface
drains is one of the main sources of nitrate in streams and rivers in the Midwest.
Concern about hypoxia, or low oxygen, in the Gulf of Mexico
has increased concern about nitrate sources. Concentrations of nitrate in tile
drains are usually quite high (10-40 mg/l).

Pesticides also flow into
subsurface drains, but only in very limited concentrations. Pesticides move
more easily in flow over the soil than through the soil, so the highest
concentrations of pesticides in tiles are often in fields that have direct
surface inlets to the drains. Subsurface drainage may reduce pesticide loss to
rivers and streams because it reduces surface runoff.

What can be done to minimize the impact of drainage on water quality?

Traditionally, the goal of
drainage design was to maximize benefits to the crop while minimizing costs of
drainage installation. Reducing water quality effects of drainage should become
a consideration in future drainage improvements. Nitrate is the biggest water
quality concern related to tile drainage, and several new technologies are
being developed that show promise for reducing negative impacts. Controlled
drainage is a system that keeps the water table in the field high during the
off-season when crops are not growing. It therefore increases the rate of denitrification (a process that converts nitrate to
harmless nitrogen gas when the soil is saturated) and reduces nitrate loss to
the environment. Controlled drainage can be combined with subirrigation to
improve yields while protecting water quality. Subirrigation
is irrigation through the subsurface drain tiles, rather than more conventional
methods such as using sprinklers. Subirrigation is often economical when fields
would need to be drained anyway, since additional infrastructure consists
mainly of increased numbers of tiles and the pumping system. One system being
developed in Ohio combines a
wetland for water treatment and a pond serving as a reservoir for subirrigation
with a drainage system. This system has been shown to increase yields and
reduce water quality impacts of drainage. It remains costly however, because of
the land that is needed for the wetland and pond/reservoir.

Conclusion

Agricultural drainage is an
essential management practice on many Indiana
soils. Appropriate drainage improves crop growth and the efficient use of
production inputs, thereby ensuring a more dependable supply of food and feed
from Indiana farms. The current
focus of most drainage improvements is to optimize crop production on land that
is already in agricultural production. Although drainage improvements were
traditionally evaluated solely for their impacts on the field in which they
were installed, it is now important to consider the impacts of further drainage
improvements on downstream water quantity and quality. Drainage affects the
entire watershed and must be considered as one element in overall water
management within the watershed.

For more information

More information on the topics
covered in this introduction can be found in the following publications:

AY-300: Drainage Recommendations
for Indiana Soils

AY-301: Wet Soils of Indiana

Ohio State University Bulletin 871
“Agricultural Drainage: Water Quality Impacts and Subsurface Drainage Studies
in the Midwest” provides more extensive information on
water quality and practices to reduce nitrate from subsurface drain tiles.

Any of these publications can be
obtained from your county office of Purdue Extension, or from the PurdueExtensionMediaDistributionCenter (1-888-EXT-INFO or
Media.Order@ces.purdue.edu.)

Acknowledgements

Thanks to the following reviewers:
Don Franzmeier, Department of Agronomy, and Bernard Engel, Department of
Agricultural and Biological Engineering. Figure 1 is based on a graph from
Skaggs, W., 1987. Principles of Drainage. In Pavelis, G . (Editor), Farm
Drainage in the United States:
History, Status, and Prospects. Economic Research Service,
USDA Misc. Pub. 1455.